Three dimensional fiducial

ABSTRACT

A method and system for forming and using a fiducial on a sample to locate an area of interest on the sample, the method comprising forming a fiducial by depositing a block of material on a sample proximal to an area of interest on the sample, the block of material extending from the surface of the sample to a detectable extent above the surface of the sample; and milling, using a charged particle beam, a predetermined pattern into at least two exposed faces of the block of material; subsequent to forming the fiducial, detecting the location of the area of interest by detecting the location of the fiducial; and subsequent to detecting the location of the area of interest, imaging or milling the area of interest with a charged particle beam.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to charged particle beam imaging andmilling.

BACKGROUND OF THE INVENTION

Charged particle beams, laser beams, and neutral particle beams are usedin a variety of microfabrication applications, such as fabrication ofsemiconductor circuitry and microelectromechanical assemblies. The term“microfabrication” is used to include creating and altering structureshaving dimensions of tens of microns or less, including nanofabricationprocesses. “Processing” a sample refers to the microfabrication ofstructures on that sample. As smaller and smaller structures arefabricated, it is necessary to direct the beam more precisely.

One method of accurately positioning a beam is to place or mill afiducial, that is, a reference mark, on the sample near an area ofinterest, and position the beam relative to the fiducial. The termfiducial is used broadly to include any type of reference mark. A beamis initially directed to image a fiducial and an initial offset to thedesired location is determined. Subsequently, the beam is periodicallydirected to image the fiducial and the positioning of the beam to thedesired location is corrected by determining an offset between theobserved coordinates of the fiducial and the original coordinates of thefiducial. The offsets are then added to the beam positioninginstructions so that the beam ends up at the desired location.

FIG. 1A shows a top view of a sample 100 including an area of interest102 and a fiducial 104. Area of interest 102 is a portion of sample 100where an imaging or milling operation is to be performed. For example,sample 100 may comprise a semiconductor wafer and area of interest 102may comprise a particular integrated circuit feature that is to beimaged by a scanning electron microscope or focused ion beam to verifywhether the feature has been manufactured to within specification.Typically, a charged particle beam is used to mill fiducial 104 in thesurface of the sample near area of interest 102 so that the area ofinterest can be located quickly and easily among a surface having manydifferent features. When subsequently imaged at an angle that is normalto the surface of sample 100 (“top down view”), fiducial 104 can be morereadily or quickly identified by the operator of the instrument (or byautomation software controlling the instrument) than the area ofinterest 102 itself can be identified.

However, fiducial 104 becomes less identifiable as fiducial 104 isimaged at angles that are not normal to the top surface of sample 100.For angles that are nearly parallel to the top surface of the sample100, fiducial 104 may not be identifiable at all. The top surface ofsample 100 is the surface that is opposite of the surface that is incontact with the sample stage holding sample 100. FIG. 1B shows a sideview of sample 100 in which fiducial 104 cannot be seen. Positioning thebeam for imaging or milling at this angle would require a secondfiducial.

SUMMARY OF THE INVENTION

Embodiments of the present invention are directed to a method forforming and using a fiducial on a sample to locate an area of intereston the sample. Embodiments of the method include forming a fiducial bydepositing a block of material on a sample proximal to an area ofinterest on the sample, the block of material extending from the surfaceof the sample to a detectable extent above the surface of the sample;and milling, using a charged particle beam, a predetermined pattern intoat least two exposed faces of the block of material; subsequent toforming the fiducial, detecting the location of the area of interest bydetecting the location of the fiducial; and subsequent to detecting thelocation of the area of interest, imaging or milling the area ofinterest with a charged particle beam.

Embodiments of the present invention are also directed to a systemcomprising at least one charged particle beam, a sample stage, a sampledisposed upon the sample stage, and a fiducial disposed upon the sample.The fiducial comprises a block of material deposited proximal to an areaof interest on the sample, the block of material extending from thesurface of the sample to a detectable extent above the surface of thesample, and a predetermined pattern milled into at least two exposedfaces of the block of material

Embodiments of the present invention are also directed to a method offorming a fiducial on a sample to locate an area of interest on thesample. Embodiments of the method include rotating the samplesubstantially ninety (90) degrees from an imaging position, subsequentto rotating the sample substantially ninety (90) degrees from theimaging position depositing a block of material on or near the area ofinterest using a charged particle beam that is directed toward thesurface of the sample at an angle that is not perpendicular to thesurface of the sample, and subsequent to depositing the block ofmaterial and prior to detecting the location of the area of interestrotating the sample substantially ninety (90) degrees to return thesample back to its initial position.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter. It should be appreciated by those skilled in the art thatthe conception and specific embodiments disclosed may be readilyutilized as a basis for modifying or designing other structures forcarrying out the same purposes of the present invention. It should alsobe realized by those skilled in the art that such equivalentconstructions do not depart from the spirit and scope of the inventionas set forth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more thorough understanding of the present invention, andadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1A shows a top view of sample 100, including area of interest 102and fiducial 104;

FIG. 1B shows a side view of sample 100;

FIG. 2A shows a top view of sample 200, including area of interest 202and three-dimensional fiducial 204;

FIG. 2B shows a side view of sample 200, including three-dimensionalfiducial 204;

FIG. 3 shows an isometric view of a 3D fiducial in accordance with oneor more embodiments of the present invention;

FIG. 4 shows a flowchart illustrating a method of forming athree-dimensional fiducial;

FIG. 5 shows a flowchart illustrating a method of using athree-dimensional fiducial to locate an area of interest on a sample;

FIG. 6 shows a micrograph of an exemplary 3D fiducial in accordance withone or more embodiments of the present invention;

FIG. 7 shows a micrograph of an exemplary 3D fiducial in accordance withone or more embodiments of the present invention;

FIG. 8 shows a side view in the “x-direction” of a 3D fiducial formed bysweeping a charged particle beam in the direction of the x-axis;

FIG. 9 shows a side view in the “y direction” of a 3D fiducial formed bysweeping a charged particle beam in the direction of the x-axis of FIG.8;

FIG. 10 shows a flowchart for a method to correct for a 3D fiducialhaving a non-orthogonal deposition profile;

FIG. 11 shows an exemplary dual beam FIB/SEM system 1110 that could beused to implement preferred embodiments of the present invention;

FIG. 12 shows a plan view of a sample 800 including an area of interest801 in an imaging position;

FIG. 13 shows a plan view of sample 800 rotated by an angle of ninetydegrees from its imaging position to its deposition position prior todepositing a fiducial;

FIG. 14 shows a plan view of sample 800 in the deposition positionincluding a deposited fiducial 804;

FIG. 15 shows a plan view of sample 800, including deposited fiducial804, rotated back into the imaging position;

FIG. 16 shows a plan view of a dual beam system 1600 in which the SEMcolumn 1606 is not orthogonal to the surface of the sample 1602, and theFIB column 1604 is orthogonal to the surface of the sample 1602; and

FIG. 17 shows a side view of the dual beam system 1600 in which the SEMcolumn 1606 is not orthogonal to the surface of the sample 1602 and theFIB column 1604 is orthogonal to the surface of the sample 1602.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Embodiments of the present invention are directed to a fiducial thatprovides accurate beam placement for the automation of imaging andmilling operations that involve more than one beam or stage position.The fiducial extends to a detectable extent into three dimensions abovea surface of a sample. The three dimensional (“3D”) fiducial is a singlereference point that can be recognized simultaneously from differentangles. Such a 3D fiducial can be used, for example, with a dual-beamsystem that includes an electron beam and an ion beam, such as theDualBeam™ family of instruments commercially available from FEI Companyof Hillsboro, Oreg., the assignee of the present application. The 3Dfiducial can be used for both electron beam image recognition and ionbeam image recognition, simplifying the procedure because fewer movingparts are required. The 3D fiducial is not limited to dual-beam systems,however, and can be used with both single beam systems as well asmultiple beam systems. The 3D fiducial can be applied to any automationrequiring image recognition, including automated Slice and View™instruments and instruments using AutoTEM™ and iFAST™ software, all ofwhich are commercially available from FEI Company.

The 3D fiducial is built by depositing a block of material on a samplenear an area of interest and then milling unique patterns into the topand sides. These patterns will have distinct brightness and contrastvalues relative to the background block material allowing for imagerecognition. At the eucentric location or height, the fiducial can berecognized from one or more beams as well as from various stage tilt androtation positions. Eucentric height is the height of the specimen atwhich its image does not moved laterally as a function of specimen tilt.The 3D fiducial allows for FIB cut placement using image recognition onan almost vertical plane. Relative to the incident beam, image and cutplacement have traditionally been done with a top down fiducial. Theprior art top down approach is not possible with milling at glancingangles (i.e., angles that are nearly parallel to the surface of thesample that is being milled or imaged) due to the almost vertical viewthe FIB has while cutting down the plane. For example, a fiducial markcan be cut into the top and side of a raised Platinum pad allowing ionbeam image recognition from this glancing angle. Using a 45° pre-tiltstub allows beam image recognition from additional angles.

The 3D fiducial can be used with any automated application that requiresa fiducial. When the automation requires one or more beams and one ormore stage positions, the 3D fiducial provides a single reference pointsolution. The 3D fiducial is particularly useful in charged particlebeam applications, such as scanned electron microscopy (SEM),transmission electron microscopy (TEM), scanning transmission electronmicroscopy (STEM), and imaging and milling with focused ion beam (FIB)systems.

Turning now to the drawings, FIG. 2A shows a top view of sample 200,including area of interest 202 and 3D fiducial 204. FIG. 2B shows a sideview of sample 200, including 3D fiducial 204. Area of interest 202 is aportion of sample 200 where an imaging or milling operation is to beperformed. Sample 200 may include, but is not limited to, asemiconductor wafer including integrated circuit features, a fixedbiological sample, or bulk material for transmission electron microscopy(TEM) sample preparation. For example, sample 100 may comprise asemiconductor wafer and area of interest 102 may comprise a particularintegrated circuit feature that is to be imaged by a scanning electronmicroscope or focused ion beam to verify whether the feature has beenmanufactured to within specification. In applications such as these, thelargest dimension of area of interest 202 may be less than onemicrometer (1 μm). In one embodiment of the present invention, thelargest dimension of 3D fiducial 204 is no greater than 100 μm. Inanother embodiment of the present invention, the largest dimension of 3Dfiducial 204 is no greater than 10 μm. In yet another embodiment of thepresent invention, the largest dimension of 3D fiducial 204 is nogreater than 1 μm.

3D fiducial 204 extends to a detectable extent into three dimensionsabove a sample surface that is to be imaged or milled. The term “above”as used herein means extending in a direction away from the interior ofthe sample material and toward empty space. The term “detectable” meanscapable of being resolved using a charged particle beam system. Because3D fiducial 204 extends to a detectable extent into three dimensionsabove the sample surface, 3D fiducial 204 is viewable by a chargedparticle beam directed toward sample 200 at a much wider set of anglesthan a fiducial that is substantially coplanar with the top surface ofsample 200 or milled down into the surface of sample 200. The chargedparticle beam includes, but is not limited to, an electron beam, an ionbeam, and a laser beam. 3D fiducial 204 is viewable from any angle thatis normal to the top surface of sample 200 (90 degrees), coplanar withthe top surface of sample 200 (0 degrees), or any angle between 0degrees and 90 degrees relative to the top surface of sample 200.Charged particle beams directed at glancing angles close to the topsurface of sample 200, for example angles less than 10 degrees, can morereadily form an identifiable image of 3D fiducial 204 because 3Dfiducial 204 extends to a detectable extent above the plane of the topsurface of sample 200.

Preferably, 3D fiducial 204 is milled with a unique, predeterminedpattern, such as predetermined pattern 206, on a least two exposed facesof 3D fiducial 204. Predetermined pattern 206 is milled to have distinctbrightness and contrast values relative to the background block materialto facilitate image recognition and location of 3D fiducial 204. Inpreferred embodiments of the present invention, image recognitionsoftware automatically locates the position of 3D fiducial 204 byanalyzing an image formed by directing a charged particle beam at thesurface of sample 200. Having a predetermined pattern on at least twofaces of 3D fiducial 204 enables 3D fiducial 204 to be simultaneouslyimaged by two or more charged particle beams and at various stage tiltand rotation positions.

FIG. 3 shows a three dimensional view of 3D fiducial 204 in accordancewith one or more embodiments of the present invention. 3D fiducial 204is located substantially proximal to area of interest 202 on sample 200.3D fiducial 204 extends to a detectable extent into three dimensions (x,y, z). 3D fiducial 204 is capable of being imaged or milled by a chargedparticle beam in at least two of the three dimensions. For example, if acharged particle beam is directed parallel to the surface of sample 200and in the y-direction, then 3D fiducial 204 is visible in at least thexz-plane. Prior art fiducials, which are marks milled in the surface ofsample 100, would be barely viewable, if at all, at angles that arenearly parallel to the surface of sample 200 because the fiducial do notextend to a detectable extent above the surface of the sample, making itdifficult or impossible to detect the location of the prior artfiducial, especially for automated image recognition software.

FIG. 4 shows flowchart illustrating a method of forming athree-dimensional fiducial. 3D fiducial 204 is formed by depositing ablock of material on sample 200 proximal to area of interest 202 (step404). The block of material is deposited such that it extends from thesurface of the sample to a detectable extent above the surface of thesample. The block of material is deposited such that it is capable ofbeing imaged or milled by a charged particle beam in at least twodimensions when the charged particle beam is directed at an angle withina set of angles that range between an angle that is coplanar with thesample surface that is being imaged or milled and an angle that isnormal to the sample surface that is being imaged or milled. Any methodfor forming a deposit on a sample may be used. For example, thedecomposition of a precursor gas in the presence of a charged particlebeam can be used to deposit the block of material on sample 200. Acharged particle beam is used to mill a predetermined pattern into atleast two exposed faces of the block of material (step 406). The formedfiducial comprises the block of material milled with the predeterminedpattern.

FIG. 5 shows flowchart illustrating a method of using athree-dimensional fiducial to locate an area of interest on a sample.After forming 3D fiducial 204 using the method shown in FIG. 4, thelocation of area of interest 202 is detected by detecting the locationof 3D fiducial 204. In one or more embodiments of the present invention,the location of the fiducial is detected by scanning a charged particlebeam across at least a portion of the surface of sample 200 and formingan image of the surface of sample 200 (step 504). In at least oneembodiment of the present invention, 3D fiducial 204 is positioned at alocation where 3D fiducial 204 can be simultaneously imaged by at leasttwo charged particle beams. 3D fiducial 204 is identified in an image ofthe sample surface by detecting the predetermined pattern 206 of 3Dfiducial 204 in the image (step 506). In at least one embodiment, anoperator of the charged particle beam instrument manually monitors theimage of the surface of sample 200 until the operator detects an imageof 3D fiducial 204, including the predetermined pattern milled on one ormore of the exposed faces of 3D fiducial 204, in the image of thesurface of sample 200. In a preferred embodiment of the presentinvention, a computer automatically analyzes the image of the surface ofsample 200 and automatically detects predetermined pattern 206 offiducial 204, for example, by detecting distinct brightness and/orcontrast values associated with predetermine pattern 206. The locationof area of interest 202 is determined based on the location of detectedpredetermine pattern 206 and 3D fiducial 204 (step 508). Afterdetermining the location of area of interest 202 in step 508, area ofinterest 202 is imaged or milled with a charged particle beam (step510).

FIG. 6 shows a micrograph of an exemplary 3D fiducial formed inaccordance with one or more embodiments of the present invention.Micrograph 600 was generated by directing an electron beam toward thesurface of sample 200 at an angle of approximately fifty-two (52)degrees with respect to a normal to the top surface of sample 200. 3Dfiducial 204 is deposited proximal to area of interest 202.Predetermined pattern 206 a is milled into the top surface of 3Dfiducial 204. The “top surface” is the surface of 3D fiducial 204 thatis substantially parallel to the surface on which 3D fiducial 204 isdesposited but is not in contact with the sample 200. Predeterminedpattern 206 b is milled into a side surface of 3D fiducial 204. A “sidesurface” is a surface of 3D fiducial 204 that is substantiallyperpendicular to the surface on which 3D fiducial 204 is desposited. Assuch, 3D fiducial 204 has a predetermined pattern 206 a-b milled into atleast two of its exposed surfaces, in accordance with step 406 of FIG.4. The distinct brightness and contrast values of predetermined patterns206 a-b relative to the background block material of 3D fiducial 204make 3D fiducial 204 more readily identifiable than area of interest202. After locating area of interest 202 using 3D fiducial 204, trench602 was milled around area of interest 202.

FIG. 7 shows another micrograph of an exemplary 3D fiducial inaccordance with one or more embodiments of the present invention.Micrograph 700 was generated by directing an electron beam toward thesurface of sample 200 at an angle of approximately ninety (90) degreeswith respect to a normal to the top surface of sample 200. 3D fiducial204 is deposited proximal to area of interest 202. A predeterminedpattern (not shown) is milled into the top surface of 3D fiducial 204.Predetermined pattern 206 b is milled into a side surface of 3D fiducial204. 3D fiducial 204 has a predetermined pattern milled into at leasttwo of its exposed surfaces, in accordance with step 406 of FIG. 4, eventhough only predetermined pattern 206 b on the side surface of 3Dfiducial is imaged by the charged particle beam. This is because thecharged particle beam is directed at a glancing angle, in this caseparallel to the top surface of sample 200, and the top surface of 3Dfiducial 204 is not visible. A prior art fiducial, milled into the topsurface of sample 200, would not be visible to a charged particle beamdirected toward the surface of sample 200 at glancing angle for the samereason the predetermined pattern on the top surface of 3D fiducial isnot visible. That is, surface features visible in a top-down or planview are not readily visible, if at all, in a side or elevation view.However, because 3D fiducial extends to a detectable extent into threedimensions above a sample surface that is to be imaged or milled, theside of 3D fiducial, including predetermined pattern 206 b, is visibleto a charged particle beam directed toward the surface of sample 200 atglancing angle. That is, 3D fiducial 204 is visible in both thetop-down/plan view and the side/elevation view, as well as any angle inbetween.

In some instances it is desirable to deposit the block of material usinga charged particle beam that is directed toward the surface of sample200 at an angle that is not orthogonal to the sample surface. Forexample, it is not possible to tilt the stage on some dual beamplatforms to produce an orthogonal deposition edge with a scanningelectron microscope (SEM). Additionally, when using small dual beamsystems having stage tilt capability, an operator might prefer to avoidstage tilt when doing SEM deposition. However, creating an“x-direction”3D fiducial (near zero damage) with a non-orthogonal SEM beam produces anon-orthogonal deposition profile, relative to the wafer surface, alongthe “x-direction”. When the subsequent deposition is imaged with the FIBbeam, normal to the wafer surface, the “z-direction” or “top down view”deposition profile is orthogonal in the “y-direction”, but not in the“x-direction”. It is critical in automated “zero-damage” applications toimage an area of interest and simultaneously create a fiducialaccurately marking location of the area of interest in the“y-direction”.

FIG. 12 shows a plan view of a sample 800 including an area of interest801 in an imaging position. The imaging position is the position of thesample used when imaging the area of interest with a charged particlebeam. In this example, a 3D fiducial is to be deposited over area ofinterest 801 using a non-orthogonal charged particle beam. Because thecharged particle beam is directed at an angle that is not orthogonal tothe surface of sample 800, the deposition profile of the fiducial is notorthogonal to the sample surface in the direction of the chargedparticle beam. To compensate, one or more embodiments of the presentinvention rotates sample 800 ninety degrees (+90°) from its imagingposition to a deposition position before depositing the fiducial. FIG.13 shows a plan view of sample 800 rotated by an angle of ninety degreesfrom its imaging position to its deposition position prior to depositinga fiducial.

FIG. 14 shows a plan view of sample 800 rotated into the depositionposition and including a deposited 3D fiducial 804. After rotating thesample 800 by an angle of ninety degrees from its imaging position toits deposition position, charged particle beam 802 is directed at thesurface of sample 800 over area of interest to deposit fiducial 804.Charged particle beam 802 is preferably scanned in a raster pattern.That is, charged particle beam 802 is scanned in rows from left to rightin the x-direction, and from top to bottom in the y-direction. Becausecharged particle beam 802 is not orthogonal to the sample surface in 800in the x-direction, the deposition profile of the fiducial is notorthogonal to the sample surface in the x-direction. Because chargedparticle beam 802 is orthogonal to the sample surface in 800 in they-direction, the deposition profile of the fiducial is orthogonal to thesample surface in the y-direction. By rotating the sample 800 from theimaging position to the deposition position prior to depositing thefiducial, the fiducial can be accurately placed with respect to they-direction because the deposition profile of the fiducial is orthogonalto the sample surface in the y-direction. This is demonstrated ingreater detail with the side views of FIGS. 8-9.

FIG. 15 shows a plan view of sample 800, including deposited 3D fiducial804, rotated back into the imaging position. After depositing fiducial804, sample 800 is rotated ninety (−90°) degrees back to its imagingposition. Sides 806 and 808 of fiducial 804 are not orthogonal to thesurface of sample 800 and are not well-suited for accurate placementrelative to area of interest 801. Sides 906 and 908 are orthogonal tothe surface of sample 800 and are well-suited for accurate placementrelative to area of interest 801.

FIG. 8 shows a side view of the “x-direction” profile of 3D fiducial 804in the deposition position. Charged particle beam 802 is scanned in araster pattern, preferably in the presence of a deposition precursorgas, to deposit a block of material for 3D fiducial 804 on sample 200.Charged particle beam 802 first forms side 806 and finishes with side808. Because charged particle beam 802 is directed at an angle that isnot orthogonal to the surface of sample 200 in the x-direction, sides806 and 808 of 3D fiducial 804 are also not orthogonal to the surface ofsample 200 in the x-direction. Sides 806 and 808 of 3D fiducial 804,because they are not orthogonal to the surface of sample 200 in thex-direction, are not suitable for marking a location of an area ofinterest in the x-direction. That is, sides 806 and 808 are “leaning” inthe x-direction so that the top of 3D fiducial 804 has a differentlocation in the x-direction than the bottom of 3D fiducial 804.

FIG. 9 shows a side view of the “y-direction” profile of 3D fiducial 804in the deposition position. Charged particle beam (not shown) is scannedin a raster pattern, preferably in the presence of a depositionprecursor gas, to deposit a block of material for 3D fiducial 804 onsample 200. Charged particle beam 802 is scanned from side 906 towardside 908, then is turned to side 906 and move slightly out of the planeof FIG. 9 to form another row. Because charged particle beam 802 isdirected at an angle that is orthogonal to the surface of sample 200 inthe y-direction, sides 906 and 908 of 3D fiducial 804 are alsosubstantially orthogonal to the surface of sample 200. Sides 906 and 908of 3D fiducial 804, because they are substantially orthogonal to thesurface of sample 200, are suitable for marking a location of an area ofinterest. That is, sides 906 and 908 are not “leaning” in they-direction so that the top of 3D fiducial 804 has substantially thesame location in the y-direction as the bottom of 3D fiducial 804.

FIG. 10 shows a flowchart for a method to correct for a 3D fiducialhaving a non-orthogonal deposition profile. Sample 200 is initiallyrotated ninety (90) degrees relative to the imaging position, the stageorientation used to image area of interest 202 (step 1004). Afterrotating the sample ninety degrees (+90°) into the deposition position,the deposition of the block of material is created in the y-direction(step 1006). The sample is then rotated ninety degrees in the oppositedirection (−90°) back to its original orientation, the imaging position,(step 1008) and imaged with the FIB or SEM (step 1010). The depositioncenter is the center of the 3D fiducial and not a function of depositiontime or thickness. The x-direction 3D fiducial now has a z-directionorthogonal profile.

FIG. 11 shows an exemplary dual beam FIB/SEM system 1110 that could beused to implement preferred embodiments of the present invention. Whilereference is made to a dual beam system, aspects of the presentinvention are not limited to a dual beam system, but may also beimplemented in other charged particle beam systems, such as single beamsystems. One embodiment of the present invention utilizes a dual beamFIB/SEM system 1110 that uses an ion beam that is either normal ortilted by a few degrees to the plane of the sample surface and anelectron beam having an axis that is also tilted, e.g., fifty-two (52)degrees from the axis of ion beam. In some embodiments, the ion beam andelectron beam are capable of aligning so that the fields of view of bothbeams are coincident to within a few microns or less. The ion beam istypically used to image and machine the work piece, and the electronbeam is used primarily for imaging but can also be used for somemodification of the work piece. The electron beam will typically producean image of a higher resolution than the ion beam image, and it will notdamage the viewed surface like the ion beam. The image formed by the twobeams can look different, and the two beams can therefore provide moreinformation than a single beam.

Such a dual beam system could be made from discrete components oralternatively, could be derived from a conventional device such as anAltura™ or an Expida™ system available from FEI Company of Hillsboro,Oreg. The present invention could also be implemented using otherparticle beam systems, including for example, single beam systems, suchas FIB or SEM only systems, or dual beam systems having two FIB columns.

Focused ion beam system 1110 includes an evacuated envelope 1111 havingan upper neck portion 1112 within which are located an ion source 1114and a focusing column 1116 including extractor electrodes and anelectrostatic optical system. Ion beam 1118 passes from ion source 1114through column 1116 and between electrostatic deflection meansschematically indicated at 1120 toward sample 1122, which comprises, forexample, a semiconductor device positioned on movable X-Y-Z stage 1124within lower chamber 1126. An ion pump or other pumping system (notshown) can be employed to evacuate neck portion 1112. The chamber 1126is evacuated with turbomolecular and mechanical pumping system 1130under the control of vacuum controller 1132. The vacuum system provideswithin chamber 1126 a vacuum of between approximately 1×10−7 Torr and5×10−4 Torr. If an etch assisting, an etch retarding gas, or adeposition precursor gas is used, the chamber background pressure mayrise, typically to about 1×10−5 Torr.

High voltage power supply 1134 is connected to ion source 1114 as wellas to appropriate electrodes in focusing column 1116 for forming an ionbeam 1118 and directing the same downwardly. Deflection controller andamplifier 1136, operated in accordance with a prescribed patternprovided by pattern generator 1138, is coupled to deflection plates 1120whereby beam 1118 may be controlled to trace out a corresponding patternon the upper surface of sample 1122. In some systems the deflectionplates are placed before the final lens, as is well known in the art.

The ion source 1114 typically provides a metal ion beam of gallium,although other ion sources, such as a multicusp or other plasma ionsource, can be used. The ion source 1114 typically is capable of beingfocused into a sub one-tenth micron wide beam at sample 1122 for eithermodifying the sample 1122 by ion milling, enhanced etch, materialdeposition, or for the purpose of imaging the sample 1122. A chargedparticle multiplier 1140 used for detecting secondary ion or electronemission for imaging is connected to signal processor 1142, where thesignal from charged particle multiplier 1140 are amplified, convertedinto digital signals, and subjected to signal processing. The resultingdigital signal is to display an image of sample 1122 on the monitor1144.

A scanning electron microscope 1141, along with power supply and controlunit 1145, is also provided with the FIB/SEM system 1110. An electronbeam 1143 is emitted from a cathode 1152 by applying voltage betweencathode 1152 and an anode 1154. Electron beam 1143 is focused to a finespot by means of a condensing lens 1156 and an objective lens 1158.Electron beam 1143 is scanned two-dimensionally on the specimen by meansof a deflection coil 1160. Operation of condensing lens 1156, objectivelens 1158, and deflection coil 1160 is controlled by power supply andcontrol unit 1145.

Electron beam 1143 can be focused onto sample 1122, which is on movableX-Y-Z stage 1124 within lower chamber 1126. Scanning electron microscope1141 produces a finely focused electron beam 1143, which is scannedacross the surface of the structure, preferably in a raster pattern.When the electrons in the electron beam 1143 strike the surface of workpiece 1122, secondary electrons and backscattered electrons are emitted.Respectively, these electrons are detected by secondary electrondetector 1140 or backscattered electron detector 1162. The analog signalproduced either by secondary electron detector 1140 or backscatteredelectron detector 1162 is amplified and converted into a digitalbrightness value by signal processor unit 1142. The resulting digitalsignal can be displayed as an image of sample 1122 on the monitor 1144.

A door 1170 is opened for inserting sample 1122 onto stage 1124, whichmay be heated or cooled, and also for servicing an internal gas supplyreservoir, if one is used. The door is interlocked so that it cannot beopened if the system is under vacuum. The high voltage power supplyprovides an appropriate acceleration voltage to electrodes in ion beamcolumn 1116 for energizing and focusing ion beam 1118.

A gas delivery system 1146 extends into lower chamber 1126 forintroducing and directing a gaseous vapor toward sample 1122. U.S. Pat.No. 5,851,413 to Casella et al. for “Gas Delivery Systems for ParticleBeam Processing,” assigned to the assignee of the present invention,describes a suitable gas delivery system 1146. Another gas deliverysystem is described in U.S. Pat. No. 5,435,850 to Rasmussen for a “GasInjection System,” also assigned to the assignee of the presentinvention. For example, iodine can be delivered to enhance etching, or ametal organic compound can be delivered to deposit a metal.

System controller 1119 controls the operations of the various parts ofdual beam system 110. Through system controller 119, a user can causeion beam 1118 or electron beam 143 to be scanned in a desired mannerthrough commands entered into a conventional user interface (not shown).System controller 119 can also comprise computer-readable memory 1121and may control dual beam system 110 in accordance with data orprogrammed instructions stored in memory 1121. CAD data concerning thesample/semiconductor stored in memory 1121 can be used to create a CADpolygon overlay or other positional data used to locate a feature ofinterest and alignment points or transfer fiducials as described above.

In alternative embodiments the FIB column is aligned so that the ionbeam is directed orthogonally to the sample surface and the SEM columnis aligned so that the electron beam is directed at an angle that is notorthogonal to the sample surface. For example, in accordance withembodiments of the present invention described in FIGS. 8-10 and 12-15,the SEM may preferably be aligned so that the electron beam is directedat a forty-five degree(45°) angle to the surface of the sample. FIGS.16-17 show illustrations of alternative embodiments the FIB column isaligned so that the ion beam is directed orthogonally to the samplesurface and the SEM column is aligned so that the electron beam isdirected at an angle that is not orthogonal to the sample surface. FIG.16 shows a plan view of a dual beam system 1600 in which the SEM column1606 is not orthogonal to the surface of the sample 1602, and the FIBcolumn 1604 is orthogonal to the surface of the sample 1602. FIG. 17shows a side view of the dual beam system 1600 in which the SEM column1606 is not orthogonal to the surface of the sample 1602 and the FIBcolumn 1604 is orthogonal to the surface of the sample 1602.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the invention as defined by the appended claims. Moreover, thescope of the present application is not intended to be limited to theparticular embodiments of the process, machine, manufacture, compositionof matter, means, methods and steps described in the specification. Asone of ordinary skill in the art will readily appreciate from thedisclosure of the present invention, processes, machines, manufacture,compositions of matter, means, methods, or steps, presently existing orlater to be developed that perform substantially the same function orachieve substantially the same result as the corresponding embodimentsdescribed herein may be utilized according to the present invention.Accordingly, the appended claims are intended to include within theirscope such processes, machines, manufacture, compositions of matter,means, methods, or steps.

We claim as follows:
 1. A method of forming and using a fiducial on asample to locate an area of interest on the sample, the methodcomprising: forming a fiducial by: depositing a block of material on asample proximal to an area of interest on the sample, the block ofmaterial extending from the surface of the sample to a detectable extentabove the surface of the sample; and milling, using a charged particlebeam, a predetermined pattern into at least two exposed faces of theblock of material; subsequent to forming the fiducial, detecting thelocation of the area of interest by detecting the location of thefiducial; and subsequent to detecting the location of the area ofinterest, imaging or milling the area of interest with a chargedparticle beam.
 2. The method of claim 1, in which detecting the locationof the fiducial includes: imaging at least a portion of the surface ofthe sample using a charged particle beam; and detecting thepredetermined pattern of the fiducial in the image of the portion of thesample.
 3. The method of claim 2, in which the predetermined pattern ofthe fiducial in the image of the portion of the sample is automaticallydetected by image recognition software.
 4. The method of claim 1, inwhich the predetermined pattern is milled into the fiducial so that thefiducial will have a brightness or contrast value that is distinct fromthe block of material when the fiducial is imaged by a charged particlebeam.
 5. The method of claim 1, in which the deposited block of materialis capable of being imaged or milled by a charged particle beam in atleast two dimensions.
 6. The method of claim 1, in which the fiducial ispositioned at a location where the fiducial can be simultaneously imagedby at least two charged particle beams.
 7. The method of claim 1, inwhich imaging is performed by an electron beam and milling is performedby a focused ion beam.
 8. The method of claim 1, in which the depositedblock of material has a substantially parallelepiped geometry.
 9. Afiducial for locating an area of interest on a sample, the fiducialcomprising: a block of material deposited proximal to an area ofinterest on a sample; the block of material extending from the surfaceof the sample to a detectable extent above the surface of the sample;and a predetermined pattern milled into at least two exposed faces ofthe block of material.
 10. The fiducial of claim 9, in which the largestdimension of the fiducial is no greater than one hundred micrometers(100 μm).
 11. The fiducial of claim 9, in which the largest dimension ofthe fiducial is no greater than ten micrometers (10 μm).
 12. Thefiducial of claim 9, in which the largest dimension of the fiducial isno greater than one micrometer (1 μm).
 13. The fiducial of claim 9, inwhich the block of material comprises a raised Platinum pad.
 14. Thefiducial of claim 9, in which the predetermined pattern is milled intothe fiducial so that the fiducial will have a brightness or contrastvalue that is distinct from the block of material when the fiducial isimaged by a charged particle beam.
 15. The fiducial of claim 9, in whichthe fiducial is positioned at a location where the fiducial can besimultaneously imaged by at least two charged particle beams.
 16. Thefiducial of claim 9, in which the deposited block of material has asubstantially parallelepiped geometry.
 17. A system comprising: at leastone charged particle beam; a sample stage; a sample disposed upon thesample stage; a fiducial disposed upon the sample, the fiducialcomprising: a block of material deposited proximal to an area ofinterest on the sample; the block of material extending from the surfaceof the sample to a detectable extent above the surface of the sample;and a predetermined pattern milled into at least two exposed faces ofthe block of material.
 18. The system of claim 17, in which the largestdimension of the fiducial is no greater than one hundred micrometers(100 μm).
 19. The system of claim 17, in which the largest dimension ofthe fiducial is no greater than ten micrometers (10 μm).
 20. The systemof claim 17, in which the largest dimension of the fiducial is nogreater than one micrometer (1 μm).
 21. The system of claim 17, in whichthe block of material comprises a raised Platinum pad.
 22. The system ofclaim 17, in which the predetermined pattern is milled into the fiducialso that the fiducial will have a brightness or contrast value that isdistinct from the block of material when the fiducial is imaged by acharged particle beam.
 23. The system of claim 17, in which the fiducialis positioned at a location where the fiducial can be simultaneouslyimaged by at least two charged particle beams.
 24. The system of claim17, in which the deposited block of material has a substantiallyparallelepiped geometry.
 25. A method of forming a fiducial on a sampleto locate an area of interest on the sample, the method comprising:rotating the sample substantially ninety (90) degrees from an imagingposition; subsequent to rotating the sample substantially ninety (90)degrees from the imaging position, depositing a block of material on ornear the area of interest using a charged particle beam that is directedtoward the surface of the sample at an angle that is not perpendicularto the surface of the sample; and subsequent to depositing the block ofmaterial and prior to detecting the location of the area of interest,rotating the sample substantially ninety (90) degrees to return thesample back to its initial position.
 26. The method of claim 25, inwhich the charged particle beam is an electron beam of a scanningelectron microscope, the electron beam being directed at the samplesurface at an angle that is not orthogonal to the sample surface. 27.The method of claim 26, further comprising: subsequent to forming thefiducial, locating the area of interest by locating the fiducial anddirecting a focused ion beam toward the area of interest, the focusedion beam being directed at the sample surface at an angle that issubstantially orthogonal to the sample surface.